In order to optimize nutrition, the movement of a meal through the small intestine must be precisely controlled to ensure that there is adequate time to complete the time-demanding steps of digestion and absorption. Abdominal pain, nausea, bloating, diarrhea and malnutrition are the consequences of impaired control of intestinal transit. After a fat-containing meal, inhibitory feedback mechanisms are activated by the proximal and distal small intestine as the jejunal and ileal brake, respectively. In contrast to this focus of the postprandial gut to slow transit, much of the research efforts over the past 100 years have been directed at the peristaltic reflex, which is responsible for the acceleration of intestinal transit and are known to be mediated by 5-HT. Currently, two roles of enteric 5-HT have been established: the triggering of the peristaltic and mucosal secretory reflexes via intrinsic primary afferent neurons and gut-to-CNS and gut-to-pancreas communications via extrinsic sensory nerves. We have recently found a third role of enteric 5-HT. Specifically, 5-HT is also involved in the slowing of transit by fat via a 5-HT3 pathway that is dependent on 5-HT transmission via myenteric neurons. In this proposal, we will test our overall hypothesis that slowing of intestinal transit by fat depends on primary afferent nerves, beta-adrenergic pathway and 5-HT transmission via myenteric neurons, which in turn activates opioid neurons, that slow transit. We will test the hypotheses using pharmacologic and physiologic approaches. The results of these studies will help us to refine our hypotheses so that we can test the neuroanatomic components of this pathway using immunohistochemistry. We have developed a collaboration with 2 leading neuroscientists who will provide this project with additional expertise in immunohistochemistry. In addition, to test the role of a novel beta-adrenergic system in the slowing of intestinal transit by fat, we have developed a multi-disciplinary team approach by including a cardiologist experienced in the 13- adrenergic system. The PI has a track record of success in bench-to-bedside translational research in the area of nutrient control of intestinal transit. His experience includes the discovery of a novel, nutrient-based strategy to slow intestinal transit. This application will bring this new treatment back to basic research so that we can understand the neural pathways that underpin the slowing response to fat. The hypotheses to be tested in this project will expand our understanding of a new role for enteric 5-HT which may explain conditions such as irritable bowel syndrome and provide better understanding of the effects of drugs that are directed at 5-HT pathways. In addition, by investigating the mediators of the control of intestinal transit, we will gain knowledge that can be used to control the movement of a meal through the small intestine and, in turn, reduce symptoms and improve nutrition.